The present invention generally relates to television signal transmission systems and particularly concerns a vestigial sideband (VSB) digital television signal transmission system having reduced susceptibility to NTSC co-channel interference.
Simulcast broadcasting is a technique which has been proposed for providing high definition television services without obsoleting the large installed base of NTSC receivers. Simply put, simulcast broadcasting contemplates simultaneous transmission of identical program material encoded in two different formats over respective 6 MHz television channels. Thus, for example, a particular program may be encoded in NTSC format for transmission over a first 6 MHz television channel and in an HDTV format for transmission over a second different 6 MHz television channel. Viewers equipped only with NTSC receivers would therefore be able to receive and reproduce the program encoded in NTSC format by tuning the first channel, while viewers equipped with HDTV receivers would be able to receive and reproduce the same program encoded in HDTV format by tuning the second channel.
The foregoing, of course, contemplates the allocation of additional 6 MHz television channels for the transmission of HDTV encoded signals within a given NTSC service area. While such additional channels are generally available for this purpose, at least some of the same channels are also quite likely to be allocated for NTSC transmissions in nearby television service areas. This raises the problem of co-channel interference where HDTV and NTSC transmissions over the same channel in nearby television service areas interfere with one another. NTSC co-channel interference into a received HDTV signal is of particular concern due to the relatively large picture and color carriers characterizing an NTSC transmission. HDTV systems employing an all digital transmission standard further add to this concern, since excessive NTSC co-channel interference from a nearby transmitter could abruptly render an HDTV receiver incapable of reproducing any image rather than gradually degrading the performance of the receiver.
A number of proposed HDTV systems contemplate a transmission standard comprising a pair of amplitude modulated, double sideband components having respective suppressed quadrature carriers located in the middle of a 6 MHz television channel. While this transmission standard has certain desirable attributes, it also has a number of disadvantages. First and foremost, cross talk between the two quadrature channels can significantly degrade receiver performance unless special care is taken to avoid or compensate for the causes of such cross talk. Other forms of transmission standards, e.g. VSB transmission, are not subject to the cross talk disadvantage and are equally desirable in other respects, especially where the transmission is effected in a digital format. The problem of NTSC co-channel interference, however, remains an important consideration before such a transmission standard can be successfully employed.